1. Field of the Invention
This invention relates to microcomputers and in particular to virtual memory arrangements for microcomputers.
2. Description of the Prior Art
In the early days of digital computers, very slow main memories were used for storing data and instructions for the computer. These memories took the form of magnetic drums, magnetic tapes and mercury delay lines. Later, magnetic switch core memories were commonly used. As the size of programs increased, magnetic drums, magnetic tapes and magnetic discs were used to store the programs for subsequent calling in to the main core memory.
Miniaturization of computer architecture included memory miniaturization resulting in integrated circuit random access main memories. In many cases these were replaced by read-only memories in which were stored the instructions for the microcomputer. As the microcomputer systems grew larger and program memories were required to grow larger, the larger ROM became expensive and contained much of the program that was not needed at the moment.
This virtual architecture uses a RAM and a low cost, large read-only mass memory for the mass storage of instructions. The instructions from the mass memory can be downloaded and overlayed onto a portion of the RAM space not immediately required for program execution.